This invention pertains to a process and catalyst for the direct oxidation of olefins, such as propylene, by oxygen to olefin oxides, such as propylene oxide.
Olefin oxides, such as propylene oxide, are used to alkoxylate alcohols to form polyether polyols, such as polypropylene polyether polyols, which find significant utility in the manufacture of polyurethanes and synthetic elastomers. Olefin oxides are also important intermediates in the manufacture of alkylene glycols, such as propylene glycol and dipropylene glycol, and alkanolamines, such as isopropanolamine, which are useful as solvents and surfactants.
Propylene oxide is produced commercially via the well-known chlorohydrin process wherein propylene is reacted with an aqueous solution of chlorine to produce a mixture of propylene chlorohydrins. The chlorohydrins are dehydrochlorinated with an excess of alkali to produce propylene oxide. This process suffers from the production of a low concentration salt stream. (See K. Weissermel and H. J. Arpe, Industrial Organic Chemistry, 2.sup.nd ed., VCH Publishers, Inc., New York, N.Y., 1993, p. 264-265.)
Another well-known route to olefin oxides relies on the transfer of an oxygen atom from an organic hydroperoxide or peroxycarboxylic acid to an olefin. In the first step of this oxidation route, a peroxide generator, such as isobutane or acetaldehyde, is autoxidized with oxygen to form a peroxy compound, such as t-butyl hydroperoxide or peracetic acid. This compound is used to epoxidize the olefin, typically in the presence of a transition metal catalyst, including titanium, vanadium, molybdenum, and other heavy metal compounds or complexes. Along with the olefin oxide produced, this process disadvantageously produces equimolar amounts of a coproduct, for example an alcohol, such as t-butanol, or an acid, such as acetic acid, whose value must be captured in the market place. (Industrial Organic Chemistry, ibid., p. 265-269.)
Metal-catalyzed processes for the direct oxidation of propylene by oxygen are known. For example, U.S. Pat. No. 5,525,741 discloses the direct oxidation of propylene with oxygen in the presence of a crystalline metallosilicate, such as titanosilicate, having supported thereon a silver salt of nitric or nitrous acid. This patent is silent with respect to conducting the process in the presence of hydrogen.
PCT publication WO-A1-96/02323 discloses the hydro-oxidation of an olefin, including propylene, with oxygen in the presence of hydrogen and a catalyst to form an olefin oxide. The catalyst is a titanium or vanadium silicalite containing at least one platinum group metal, and optionally, an additional metal selected from silver, iron, cobalt, nickel, rhenium, and gold.
The aforementioned direct oxidations employ catalysts which are deficient in activity and/or selectivity to propylene oxide.
PCT publication WO-A1-97/25143 discloses the hydro-oxidation of an olefin, including propylene, with oxygen in the presence of hydrogen and a catalyst to form the corresponding olefin oxide. The catalyst is a titanium or vanadium silicalite containing a lanthanide metal. Optionally, an additional metal selected from the Group 8 metals of the Periodic Table, rhenium, silver, and gold may be incorporated into the catalyst. Catalysts consisting of a lanthanide metal and a titanium or vanadium silicate exhibit low activity to propylene oxide.
In view of the above, a need continues to exist in the chemical industry for an efficient direct route to propylene oxide and higher olefin oxides from the reaction of oxygen with C.sub.3 and higher olefins. The discovery of such a process which simultaneously achieves high selectivity to the olefin oxide at an economically advantageous conversion of the olefin would represent a significant achievement over the prior art.
This invention is a novel process of preparing an olefin oxide directly from an olefin and oxygen and hydrogen. The process comprises contacting an olefin having at least three carbon atoms with oxygen in the presence of hydrogen and a catalyst under process conditions sufficient to produce the corresponding olefin oxide. The catalyst which is employed in the process of this invention comprises silver and titanium. In another aspect of the process of this invention, the catalyst comprising silver and titanium can further comprise gold, or at least one promoter element as noted hereinafter, or a combination of gold with one or more promoter elements.
The novel process of this invention is useful for producing an olefin oxide directly from oxygen and hydrogen and an olefin having three or more carbon atoms. Under preferred process conditions, the olefin oxide is produced in a high selectivity at a good conversion of the olefin.
In another aspect, this invention is a unique catalyst composition comprising silver, at least one promoter element, and a titanium-containing support. The promoter element is selected from Group 1, Group 2, zinc, cadmium, the rare earth lanthanides, and the actinide elements, as well as combinations of these elements.
In a third aspect, this invention is a unique catalyst composition comprising silver, gold, and a titanium-containing support. Optionally, this catalyst can contain at least one promoter element selected from Group 1, Group 2, zinc, cadmium, the rare earth lanthanides, and the actinide elements, including combinations thereof.
The novel compositions of this invention can be effectively used in the aforementioned direct oxidation of an olefin having three or more carbon atoms to the corresponding epoxide. In preferred embodiments, the catalysts achieve a high selectivity to olefin oxide at a good conversion of the olefin. When the catalyst is partially or completely spent, it is easy to regenerate. Accordingly, this composition possesses desirable properties for catalyzing the direct oxidation of propylene and higher olefins to their corresponding olefin oxides.